Study on decreasing trend of air dose rates observed in Fukushima prefecture and neighboring prefectures

Car borne surveys in wide area (13 prefectures in east Japan) has been performed by Japan Atomic Energy Agency (JAEA) with the use of the KURAMA and KURAMA-II systems since the Fukushima Daiichi Nuclear Power Plant accident. In this report, the ratio of air dose rate averaged in prefecture-wise was investigated and distribution maps to show the ratio of air dose rate between different surveys were made, for investigating time dependency of the change of air dose rate focused in measurement area (land usage or air dose rate). The changes in air dose rate from the first (June 2011) car borne survey to the fourth (September 2012) and seventh (November 2013) car borne surveys were grouped by the magnitude of air dose rate in the first survey. It was found that the ratios between the air dose rate of the first to the fourth and the seventh in Fukushima Prefecture was similar to those in Tochigi Prefecture, and the ratio in Miyagi Prefecture showed quicker decay than those in the other prefectures in the groups of less than 0.5 μSv/h. Distribution maps of the ratios of air dose rate obtained in 100 m mesh wise in Fukushima, Tochigi and Miyagi Prefectures showed that the ratios (i.e. decreasing rates) were depending on the area or road. Further, decreasing in the air dose rate was quicker than the physical decay in the big cities where population is big and the traffic is huge. (author)